In FIG. 8 is shown a schematic view of a conventional supervisory apparatus for an oil-filled electrical equipment, which has been disclosed in, JP-B-6111054. In this Figure, reference numeral 1 designates a transformer, reference numeral 2 designates an oil-filled enclosure which has, e.g., oil as an insulating medium for the transformer 1 therein, reference numeral 3 designates a pressure releasing pipe which breaks a pressure releasing plate 4 to remove the oil outside so as to depressurize the inside of the oil-filled enclosure 2 when an abrupt change in pressure occurs in the enclosure 2, reference numeral 5 designates a pressure detector which is mounted to the oil-filled enclosure 2 to detect a change in the pressure of the insulating medium, reference numeral 6 designates a memory where changes in pressure during past trouble are stored, reference numeral 7 designates a processing unit which finds pressure and time characteristics from now on based on a pressure value given by the pressure detector 5 and the past information given by the memory 6, reference numeral 8 designates a determination processor which determines the presence and absence of trouble based on the calculation results given by the processing unit 7, and reference numeral 9 designates a relay circuit which operates depending on the calculation results given by the determination processor 8. Although a portion of the pressure releasing pipe 3 which is located on the side of the oil-filled enclosure 2 with respect to the pressure releasing plate 4 is filled with the oil, the remaining portion of the pressure releasing pipe 3 on the other side is open to the atmosphere.
The operation of the conventional apparatus will be explained. When internal short circuit has occurred in an oil-filled electrical equipment such as a transformer, the short-circuited portion generates heat to and decomposes the oil sealed as the insulating medium. If a portion of the oil evaporates, the hydraulic pressure in the oil-filled enclosure 2 could increase to break the oil-filled enclosure 2, causing leakage of oil and a fire. In order to cope with this problem, a rise in the hydraulic pressure is detected, and when the hydraulic pressure becomes high, the pressure releasing plate 4 is intentionally broken to release oil through the pressure releasing pipe 3, depressurizing the inside of the oil-filled enclosure 2.
The releasing operation will be described. First, the pressure in the oil-filled enclosure 2 is detected by the pressure detector 5. Only pressure values among the detected pressure values, which are beyond a certain threshold value, are inputted into the processing unit 7. This is because minute changes in pressure which occur in normal operation are ejected from being used as data for determination. Next, the processing unit 7 finds changes in pressure and time characteristics based on the inputted pressure values and the changes in pressure based on past trouble stored in the memory 6, and finds a time required for achieving an allowable pressure value against breakage of the oil-filled enclosure 2 or a pressure value which is attained in a certain period of time. The determination processor 8 compares the calculation results of the processing unit 7 with an initial set value, which corresponds to the time required for achieving the allowable pressure value, against breakage of the oil-filled enclosure 2 or the pressure value which is attained in such a certain period of time. If the found time or pressure value meets with the initial set value, it is determined that trouble has occurred, and the relay circuit 9 is activated to carry out the pressure releasing operation.
Since the conventional supervisory apparatus for an oil-filled electrical equipment is constructed as stated above, the pressure releasing operation is carried out when a detected pressure value is beyond a certain pressure value (absolute pressure of 2 kg/cm.sup.2) and when the detected pressure value has achieved the certain pressure value in a short period of time (e.g., 1 kg/cm.sup.2 /sec). Although it is reasonable for the conventional apparatus to take action when an internal short circuit has occurred, the conventional apparatus has created a problem in that the pressure releasing operation is often carried out, even by a factor irrelevant to an internal short circuit, such as external short circuit, to improperly break the pressure releasing plate 4. If the pressure releasing plate is improperly broken, a large amount of expense is required for replacement of the broken pressure releasing plate, and it is necessary to find the cause of the improper pressure releasing operation.